Conventionally, a bearing is known to be used in a mechanical device such as a planetary gear speed reducer. The bearing is used in such a manner that the outer ring thereof is rotated. In this case, since the load region on the bearing shaft positioned on the inner ring is always at the same position, there is such a problem that the operation life of the bearing shaft is shortened due to fatigue peeling.
In order to deal with such a problem, for example, Japanese Patent Laying-Open No. 2015-7265 discloses a technique for forming a hardened layer on the surface of a bearing shaft. According to the technique, after a bearing shaft is made of alloy steel containing 0.1 to 0.5% by mass of carbon and other alloy elements, it is subjected to a carbonitriding treatment, a high-frequency quenching treatment and a tempering treatment, and thereafter, a shot peening treatment is performed on the bearing shaft so as to form a hardened layer on the surface of the bearing shaft.
According to Japanese Patent Laying-Open No. 2015-7265, it is possible to sufficiently increase the surface hardness of the bearing shaft by forming the hardened layer mentioned above on the surface of the bearing shaft, and to improve the rolling fatigue strength or the peeling resistance of the bearing shaft by imparting compressive residual stress into the hardened layer.